In the processing of semiconductor integrated circuits a layer of a dielectric material, such as silicon dioxide, will often be formed to cover and protect many of the electronic devices formed in the semiconductor material. A problem which arises during the manufacturing of such semiconductor processes is the contamination of such a surface dielectric with mobile ions. Sodium ions, in particular, are a common source of problems.
The deeper such mobile ions are embedded in the dielectric layer the greater is the problem associated with them. Two types of problems are particularly great. One problem is the undesirable leakage path such ions can provide between different semiconductor regions through the dielectric. The second problem is that the electric charge on such ions can act to provide an electrical bias to semiconductor regions underlying the dielectric layer, thereby shifting thresholds of semiconductor devices.
Such mobile ion contamination can come from a number of sources. For example, various metals are commonly sputtered onto the surface of the semiconductor itself, such as for contact pads, or onto the surface of the dielectric layer. Sputtering targets used to provide the metal to be sputtered commonly contain trace amounts of sodium contamination. Much of this sodium will become embedded in the dielectric material where it acts, as described above, as mobile ion contamination. Another common source of sodium contamination arises during any process involving a plasma. Various polymer materials used in the construction of plasma containers contain sodium. The operation of the plasma will leach some of this sodium out of the polymer whereby it may become implanted in the dielectric material. The increasing use of plasma etching and plasma resist stripping have greatly increased the mobile ion contamination coming from this source. A third source of mobile ion contamination arises from the photoresists used in semiconductor processing. Many of these photoresists contain trace amounts of sodium. When such photoresists are removed by ashing in an oxygen based plasma, a common technique, the sodium contained therein is deposited on the surface of the underlying dielectric.
The prior art approach to the problem of mobile ion contamination is to replace the silicon dioxide layer with a layer of a phoso-silicate glass formed by low pressure chemical vapor deposition. Each phosphorus atom in the phoso-silicate glass is capable of binding and neutralizing one sodium atom. The sodium atoms thus bound and neutralized will not contribute to the problems described above.
The problem with the prior art approach relates to the properties of the phoso-silicate glass. Among the problems related to such glasses are that they have unacceptably high defect levels as compared with silicon dioxide, they provide poor step coverage over features in the underlying semiconductor, they require high temperature processing, and may contribute to corrosion of metals. Furthermore, as stated above, phoso-silicate glasses are formed by low pressure chemical vapored deposition when, in some instances, other techniques such as thermal oxidation of a silicon wafer may be preferrable.